US Sen. Rand Paul (R) of Kentucky is seen on a screen introducing Senate minority leader and senior Kentucky Sen. Mitch McConnell, after McConnell defeated a tea-party rival on Tuesday. On Wednesday, Paul took to the Senate floor to lambast drones, a defining issue for the prospective 2016 GOP presidential candidate.

WASHINGTON — The speech didn't come close to the 13-hour filibuster of last year, but on Wednesday, Sen. Rand Paul (R) of Kentucky again took to the floor of the US Senate with impassioned words about civil liberties and US drone strikes.

The spark that set him on fire was, like last year, a nomination by President Obama. In March 2013, Senator Paul, a tea party favorite, filibustered the nomination of John Brennan to be CIA director, warning of the potential use of government drones against Americans on US soil. This time, he objected to the nomination of David Barron to the US Court of Appeals for the First Circuit over concern about targeted killings of American suspects overseas.

Mr. Barron is a Harvard law professor, but when he worked for the Justice Department, he co-wrote legal memos that defended the Obama administration’s policy of targeted killings abroad. The memos related to American Anwar al-Awlaki, the Al Qaeda operative killed by a US drone strike in Yemen in 2011. The memos reportedly defend the strike as lawful on grounds that Mr. Awlaki was a participant in the war with Al Qaeda on the enemy side and represented a specific threat to the US.

The legal material has been secret but, under pressure from senators, was recently made available to the full Senate – not just members of the Justice and Intelligence committees. On Tuesday, the Obama administration, under court order, said it would release to the public a redacted version of the material. That has satisfied one of the Democratic critics, Sen. Mark Udall of Colorado, who supported Barron's nomination in a largely party-line procedural vote on Wednesday afternoon.

Senator Udall welcomed the administration’s transparency and, in a statement, said it “affirms that although the government does have the right to keep national security secrets, it does not get to have secret law.” The material is not expected to be released before the Senate votes on Barron's nomination, probably Thursday.

But transparency wasn’t enough to satisfy Paul, a leader in GOP presidential polls. He tried, unsuccessfully, to delay Wednesday’s procedural vote on the nomination until the public fully debates the issue. As with other post-9/11 security issues – torture of terrorism suspects, unlimited detention of detainees, and widespread surveillance of American phone data – drone strikes challenge the balance between individual liberty and safety.

“Am I the only one who thinks that something so unprecedented as an assassination of an American citizen” should be discussed “in the light of day?” the senator asked, with incredulity.

Because Senate majority leader Harry Reid (D) of Nevada “nuked” the filibuster on presidential nominees last year, Paul held the floor Wednesday for only a half-hour. But he packed those 30 minutes with patriotic passion in defense of innocent-until-proven-guilty and of due process under the law. Even “evil” men and “traitors” deserve a right to trial by jury, he argued, because the Bill of Rights is meant especially to protect the “unpopular.”

Paul traveled the path of history. He invoked America’s second president, John Adams, who before he became president “defended the unpopular” British troops for their role in the Boston Massacre, and concluded with Thurgood Marshall, who “defended the unpopular” when he persuaded the US Supreme Court to strike down school segregation.

“Unpopular opinions change from generation to generation,” Paul continued. “While today it may be burqa-wearing Muslims, it has at times been yarmulke-wearing Jews” and may someday be Evangelical Christians who become a persecuted minority.

Paul made clear he is talking about American suspects abroad who are not involved in combat. He argued that the US Constitution provides for the proper way to deal with Americans abroad who refuse to return – that is, to try them in abstentia.

“If they are found guilty, the method of punishment is not the issue,” he said. “The issue is and always has been the right to a trial, the presumption of innocence, and the guarantee of due process to everyone, no matter how heinous the crime."

By speaking out on the issue, “Paul is in a no-lose situation,” says John Pitney, a congressional expert at Claremont McKenna College in Claremont, Calif. The senator can’t stop the Barron nomination from going forward, so he can’t be accused of obstructionism, says Mr. Pitney. According to Gallup, most Americans oppose drone strikes against American citizens even when they are living abroad.

“Republicans, who might rally behind a GOP president employing drones, are inclined to oppose pretty much anything that President Obama does,” Mr. Pitney said. “Democrats are, at best, ambivalent.”